1. Field of the Invention
The invention relates to a machining tool for a vehicle hub unit, and a vehicle hub unit. More specifically, the invention relates to a machining tool for forming spline teeth on an axially inner end face of a hub wheel to which a wheel is fitted, and a vehicle hub unit in which the spline teeth are formed.
2. Description of Related Art
A hub unit is used to support a wheel of a motor vehicle such that the wheel is rotatable with respect to a suspension device. As a hub unit of this type, there is a conventional hub unit in which spline teeth (side face splines), to be meshed with teeth formed on an outer ring of a constant velocity joint, are formed on an axially inner end face (an end face that faces inward in the vehicle lateral direction) of a hub wheel to which a drive wheel is fitted (refer to, for example, Published Japanese Translation of PCT Application No. 2008-536737).
Spline teeth of a hub wheel are usually formed plastically with the use of a punch (machining tool) having tooth forming portions. The punch is formed in a substantially cylindrical shape, and multiple tooth forming portions are formed on one axial end face thereof so as to be arranged in a circular pattern. Then, the tooth forming portions of the punch are pressed against an axial end face of the hub wheel to transfer the shapes of the tooth forming portions to the axial end face of the hub wheel. In this way, the spline teeth are formed.
When the tooth forming portions of the punch are pressed against the axial end face of the hub wheel to plastically form the spline teeth, large stress is produced in the tooth forming portions. If such large stress is repeatedly produced, the tooth forming portions may break due to fatigue. If the tooth forming portions break soon, the manufacturing costs, of course, increase, and a product price reflects the increase in manufacturing costs. Therefore, it is desired that stress that is produced in the tooth forming portions be reduced to increase the durability of the punch.
In order to increase the durability of the tooth forming portions of the punch, the shape of each tooth forming portion may be changed into such a shape that stress concentration is less likely to occur. However, the shapes of the tooth forming portions are directly transferred to the spline teeth. Therefore, a change in the shape of each tooth forming portion directly influences the shape of each spline tooth. Accordingly, it is extremely important to design the shape of each tooth forming portion such that stress that is produced in the tooth forming portions is reduced while ensuring sufficient strength and durability of the spline teeth.